Excerpt from The Autocar Handbook: A Guide to the Motor Car

In order to allow the cylinder to be first of all filled with gas, then to be hermetically closed whilst the piston travels inwards and compresses the gases, still to be hermetically closed, whilst the mixture explodes and drives the piston outwards, and lastly, to be opened to the air so that the products of combustion may be expelled by the inwardly travelling piston, it is necessary to provide doors or valves, two in number - one for the inlet of fresh gas and the other for the exhaustion of the products of combustion. Both inlet valve and exhaust valve only Open once in every two revolutions of the crankshaft, and it is accordingly necessary to actuate them through the medium of a half time Shaft, that is to say, a shaft driven at half the speed Of the crankshaft. The inlet v, alve it will be at once seen, is open during the suction stroke of the piston. During the next two strokes - compression and ignition - both valves are closed, whilst during the next stroke the exhaust valve is open, thus freeing a passage for the waste gases into the open air. The inlet valve is in direct communication with the carburetter already mentioned, whilst the exhaust gases are directed into a muffler or silencer, whereby the sharp explosive report of the exhaust is broken up into a less Offensive sound. The valves are generally of the mushroom type, resembling somewhat a very large French nail, the head of which is capable of making superficial contact with a ring-shaped valve seating, for which purpose the stem of the valve runs in a guide. In the normal position the valve is held on to its seating by a spring sufficiently strong to overcome the tendency of the suction stroke to open it. And also to be capable of bringing the valve very Sharply back on to its seating after it has been opened. The last named function is performed bv means of a shaft having at certain specified points of its periphery a hump or cam. Whilst the valve is closed the base of its stem bears upon the circular portion of the camshaft, which, as it rotates, brings the hump or cam under the valve stem with a wedge-like action that forces the valve Off its seat, to which it is returned at the proper time by the aforementioned Spring. Both valves are operated in this manner on all modern cars nowa days, but there are a very large number of older models, still in use at the present time, in which the inlet valve is worked automatically by the suction of the piston upon its suction stroke. The latter means, although it possesses advantages in the way of simplicity, cheapness, and, as some of its advocates still hold, other ways too, suffers from the fact that at high speeds the valve is inclined to remain open for a longer time than is really necessary.

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